Let the cat out of the bag Informal

To accidentally reveal a secret or spoil a surprise, usually through carelessness rather than intent.

It often carries a note of mild exasperation or comic regret, since the slip is rarely deliberate.

The popular explanation — that dishonest medieval market vendors sold cats in sacks after passing them off as piglets, with the fraud exposed when the bag was opened or the cat let out — is widely repeated but doesn’t hold up well under scrutiny. As fact-checkers at Snopes have pointed out, a cat and a piglet feel, sound, move, and weigh so differently that no buyer handling the sack, let alone hearing or feeling the animal inside, would likely be fooled before ever opening it, which undercuts the idea that “letting the cat out” was the moment such fraud was typically discovered. The theory is probably a later conflation with the genuinely documented related phrase “a pig in a poke” (a poke being a small sack), warning buyers not to purchase something unseen, attested in English since at least 1555. As for “let the cat out of the bag” itself, its true origin remains unresolved; reference works are candid that no single explanation is confirmed, and the phrase may simply have caught on because of the vivid, almost comic mental image of a startled cat bolting the instant a bag is opened — a fitting metaphor for a secret that, once released, cannot be called back.

He let the cat out of the bag about the surprise party before we’d even finished planning it.

Try not to let the cat out of the bag — she doesn’t know about the promotion yet.

The intern let the cat out of the bag during the meeting, mentioning the merger before it was announced.

A: How did everyone find out about the engagement so fast? B: My sister let the cat out of the bag on social media.